This invention overcomes the frequent inappropriate apparent rotation rate of the wheels of moving vehicles that commonly appear in televised, video, filmed, computer, and other media images. This inappropriate rotation is caused by the stroboscopic effect, which arises because many wheels of moving vehicles, trucks, wagons and other vehicles, are composed of, or have on them, periodic or symmetrical structures such as spokes or wheel covers hubcaps with regularly placed areas, markings, or other designs, and are seen by the viewers only at the periodic and discrete intervals given by the rapid sequence, or the frame rate, of the video display medium. This stroboscopic effect, which is equivalent to the wheels or wheel covers being illuminated only at the rapid sequence of instants given by the frame rate, frequently makes the wheels appear to rotate at rates that are unrelated to the actual rate at which they should rotate due to the motion of the vehicle. Thus the wheels are frequently seen rotating forward, backward, or not at all, bearing little correlation with the actual vehicle speed. The same effect is also apparent when basically linear objects with regular features, such as ladders, are moved across the video field of view, the ladder appearing to have inappropriate linear motion, forward, backward, or appear motionless.
The origin of the stroboscopic effect is described referring to FIGS. 1 through 4, in which:
FIG. 1 illustrates a typical car, as example of a vehicle
FIG. 2 illustrates a typical wheel assembly on a typical car, as example of a vehicle
FIGS. 3A-3L show the inappropriate apparent rotation rate of a wheel at one vehicle speed
FIGS. 4A-4L show the inappropriate apparent rotation rate of a wheel at a different vehicle speed
These figures contain numerical elements as follows:    10 Vehicle (in FIG. 1)    20 Wheel assembly (in FIGS. 1 and 2)    21 Wheel assembly with distinctive imaginary mark on tire (in FIGS. 3A-3L and 4A-4L)    22 Tire (in FIG. 2)    23 Imaginary distinctive mark on tire (in FIGS. 3A-3L and 4A-4L)    24 Rim of wheel (in FIG. 2)    25 Stationary vertical reference line (in FIGS. 3A-3L and 4A-4L)    26 Inner surface of wheel (in FIG. 2)    28 Spoke structures of wheel (in FIG. 2)    29 Hub or portion of vehicle holding the wheel (in FIG. 2)
This description begins with a discussion of a typical vehicle and the features of its wheels that give rise to the stroboscopic effect when video of the moving vehicle is presented to a viewer display, and which results in the inappropriate rotation perception problem. This is presented to facilitate understanding of the invention embodiments and their operation.
FIG. 1 shows a typical vehicle 10, including its spoked wheel assemblies 20, on the ground (which is not shown). FIG. 2 illustrates one wheel assembly, which generally comprises a rubber tire 22 mounted on a metal wheel. This metal wheel has a rim 24 and a generally cylindrical base 26 supported by spokes 28 or other structures, in this example shown as a six-sided star shape with truncated tips. This metal wheel connects the entire wheel assembly to the drive shaft or other mechanisms mounted on the vehicle suspension represented by housing 29 if the wheel assembly is driven by the vehicle's power train, or to surfaces not shown that are free to rotate if the wheel assembly is not driven by the power train of the vehicle. In some implementations the spokes of the wheel or some portions thereof are covered with a decorative plate or cap or hub cap. The particular design of the spokes or decorative cap or the like including the thickness, length, shape, color, and number of elements are unimportant except that there is usually an integer number of such spokes or design elements and they are generally regularly spaced around the wheel. It is this regular spacing of an integer number of elements which, together with their rotation rate and the images being presented to the viewer at another given rate, creates the stroboscopic effect.
FIGS. 3A through 3L illustrate the problem in a sequence of drawings of a rotating spoked wheel assembly as they would be seen if taken by a video camera and presented by the user displays at a fixed frame rate, in this example shown as 60 times per second, though the principle is the same at different frame rates. At each instant of time, in this example sequentially at every 1/60th of a second, is shown the typical wheel assembly with six spokes but drawn in simplified form as a six-pointed star, as one of many shapes a wheel or hub cap or cover can take. Thus FIG. 3A is taken at the start of the sequence of video frames, FIG. 3B is taken 1/60th of a second later, FIG. 3C is taken at 2/60th of a second later, FIG. 3D is taken at 3/60th second later, and so on with the last image, FIG. 3L, being shown taken at 11/60th of a second later.
In each of these figures the tire of the wheel assembly has imprinted on it an imaginary mark shown as circle 23 so that we may discern and discuss the actual physical rotational position of the wheel assembly, which is now identified as 21 to differentiate it from the generic wheel assembly 20 which does not have such an imaginary mark or circle shown on its tire. It must be understood that this marking of the tire is purely for illustrative purposes, and that wheel assemblies or tires do not generally have such marks nor do they need any such markings in order for the stroboscopic effect to materialize. Also shown in these figures is a vertical reference line 25, also imaginary, so that the perceived rotation of the wheel assembly can be better visualized for the purposes of this discussion, since the actual spoked part of the wheel assembly in general has no identifying marks to distinguish one of its spokes or elements from another. It must also be understood that the wheel assembly is in contact with a road or other generally horizontal surface, which is not shown in any of the illustrations.
This wheel assembly 21 is rotating clockwise, as shown by the position of the painted reference circle 23 on the tire in successive images, the rotation being shown incrementally as about 50 degrees every 1/60th of a second. The wheel assembly is generally rotating faster than the eye can track any mark on the tire. Thus the eye would not perceive the wheel assembly as rotating at all while it is actually rotating one full revolution in 7/60th of a second, as shown in FIG. 3H. This is an equivalent rotational rate of 8.57 revolutions per second, which is 514 revolutions per minute. Given that a typical vehicle wheel assembly with tire has a diameter of 0.6 meters (2 feet) or so, this corresponds to an outer circumference of the tire of 3.14 times 0.6, which is 1.9 meters (6.28 feet), which means that the car and wheel assembly would move 1.9 meters (6.28 feet) linearly during the time a wheel assembly makes one revolution. Therefore a vehicle which rotates its wheel assembly at 8.57 revolutions per second travels 8.57 times 1.9 meters (6.28 feet) in each second, which is 16.3 meters per second (53.8 feet per second). This equates to 58.7 kilometers per hour (40.4 miles per hour), which is not an unusual speed for vehicles to be shown traveling in television, videos, or movies; and in any case is only shown as an example as the stroboscopic effect can and does occur at other speeds as well.
It is seen by examination of FIGS. 3A through 3L that while the actual rotation speed of the wheel assembly is 8.57 revolutions per second clockwise, the apparent rotation of the wheel assembly 21 is slowly counterclockwise, as seen by observing the triangular spokes, which are regularly placed around the wheel assembly, going counterclockwise past the static vertical reference line 25. In the example of FIGS. 3A through 3L the spoked wheel has apparently rotated 90 degrees in 11/60th seconds, which equates to 1.36 revolutions per second counterclockwise. The eye is not capable of following the actual rapid clockwise rotation of the wheel assembly, and instead perceives that the wheel assembly rotates in a wrong direction by following the slow counterclockwise rotation. This effect is unchanged even if the reference mark were omitted, it being included in the illustration only for clarity.
Thus FIGS. 3A through 3L clearly illustrate the problem commonly experienced in video and film viewing, that often the perceived rotational speed of the wheel assembly bears little relationship to its actual rotational speed except for a few, fleeting instants of time should the vehicle be accelerating or decelerating through particular speeds when the stroboscopic effect produces the proper apparent rotation speed for the vehicle speed; and when it is stopped.
FIGS. 4A through 4L illustrate a different example in which the actual rotational speed of the wheel assembly 21 is still clockwise, but more rapid than that shown on FIGS. 3A though 3L. In this case the perceived rotation of the wheel assembly is zero as seen against the static reference vertical line because the wheel rotates an amount during 1/60th of a second that brings the symmetrical wheel spoke pattern to a new rotational position which is such that it appears identical to the previous position even though rotated. This occurs because each spoke of the wheel is identical to all other spokes, and regularly located circumferentially around the wheel. This is a second example that the perceived rotational rates of the wheel assemblies on a vehicle often bear little relationship to their actual rotational speed, if seen in a video, movie, television, or other such display using rapid periodic presentation.
These inappropriate effects are disconcerting at best for the viewer, and deleterious to manufacturers and advertisers when displaying their vehicles and other vehicles for sale to consumers. While this application addresses both rotational and translational inappropriate apparent motion the application will principally discuss the rotational problem, it being understood that all aspects and content of this patent application and its embodiments apply to both rotational and translational effects. Remarkably this artifact has not been remedied, and inappropriate rotation images are unfortunately ubiquitous, despite the fact that automobile manufacturers alone spend hundreds of millions of dollars annually for television and video advertising, a substantial fraction of which feature moving vehicles yet all of which suffer from the inappropriate apparent rotation problem. Furthermore this problem is not new. Consider that film movie projection is well over 100 years old now and commercial television is over 60 years old, and both have suffered from the same stroboscopic motion effect all the time, from backwardly rotating wagon wheels in western movie films to ubiquitous televised images of vehicles with wheels whose rotation bears little or no relation to the actual speed of the vehicle, and frequently show stopped wheels, backward-rotating wheels, or forward rotating wheels at the wrong rate on a vehicle moving rapidly forward.